Microsc. Microanal. 20, 484–492, 2014 doi:10.1017/S1431927614000488
© MICROSCOPY SOCIETY OF AMERICA 2014
In-Situ Electrochemical Transmission Electron Microscopy for Battery Research B. Layla Mehdi,1,* Meng Gu,2 Lucas R. Parent,1 Wu Xu,3 Eduard N. Nasybulin,3 Xilin Chen,3 Raymond R. Unocic,4 Pinghong Xu,5 David A. Welch,5 Patricia Abellan,1 Ji-Guang Zhang,3 Jun Liu,3 Chong-Min Wang,2 Ilke Arslan,1 James Evans,2 and Nigel D. Browning1 1
Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, 99352, USA 3 Energy and Environmental Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA 4 Oak Ridge National Laboratory, Center for Nanophase Materials Sciences, Oak Ridge, TN 37831, USA 5 Department of Chemical Engineering and Materials Science, University of California-Davis, One Shields Ave, Davis, CA 95616, USA 2
Abstract: The recent development of in-situ liquid stages for (scanning) transmission electron microscopes now makes it possible for us to study the details of electrochemical processes under operando conditions. As electrochemical processes are complex, care must be taken to calibrate the system before any in-situ/operando observations. In addition, as the electron beam can cause effects that look similar to electrochemical processes at the electrolyte/electrode interface, an understanding of the role of the electron beam in modifying the operando observations must also be understood. In this paper we describe the design, assembly, and operation of an in-situ electrochemical cell, paying particular attention to the method for controlling and quantifying the experimental parameters. The use of this system is then demonstrated for the lithiation/delithiation of silicon nanowires. Key words: in-situ liquid ec-TEM, assembly of in-situ liquid electrochemical TEM cell, Li-ion battery, Si anode, Si nanowire lithiation/delithiation, Li-ion battery
I NTRODUCTION The rapidly growing need for new energy storage materials (both electrodes and electrolytes) has created a high demand for experimental techniques that can provide real-time information on the dynamic electrochemical processes taking place at the liquid electrolyte/electrode interface during battery charge/discharge cycles. There are many in-situ techniques that have provided essential insights into the operation of batteries, such as optical microscopy (Beaulieu et al., 2001; Rosso et al., 2006), scanning electron microscopy (SEM) (Orsini et al., 1998; Chen et al., 2011a), X-ray diffraction (Hatchard & Dahn, 2004; Obrovac & Christensen, 2004; Li & Dahn, 2007), nuclear magnetic resonance spectroscopy (Key et al., 2009, 2011; Bhattacharya et al., 2010), transmission X-ray microscopy (Chao et al., 2010), Raman spectroscopy (Hardwick et al, 2008; Long et al., 2011), and neutron diffraction (Bridges et al., 2012). However, most of these techniques focus on examining the structure of either the cathode or anode materials and do not focus on the whole system (i.e., the interface between the liquid electrolytes and electrodes) that is crucial to understanding ion transport and battery performance (Liu et al., 2011; Yu et al, 2012). An in-situ liquid electrochemical transmission electron microscope holder (in-situ liquid Received November 10, 2013; accepted February 20, 2014 *Corresponding author. [email protected]
ec-TEM), on the other hand, offers unique opportunities for monitoring the dynamic processes and structural evolution of varied electrode materials and their interactions with battery-relevant electrolytes during operation at both spatial and temporal resolution (Huang et al., 2010; Wang et al., 2010; de Jonge & Ross, 2011; Gu et al., 2013; Sacci et al., 2014; Unocic et al., 2014), something that is not achievable with the previously mentioned characterization techniques. Open-cell in-situ TEM holders have already made a significant contribution into a fundamental understanding of structural and chemical changes of many electrode materials, in particular, silicon nanowires (Si NW) and tin dioxide (Wang et al., 2010; McDowell et al., 2012a, 2012b). The simple design of a nanobattery open-cell is illustrated in Figure 1. This approach is based on two gold electrodes facing each other, where the working electrode is modified with the nanomaterial of interest, e.g., a Si NW, and the counter electrode is comprised of lithium cobalt dioxide (LiCoO2), which is used to create the full cell. The nominal poor conductivity of the Si NW anode (~1–10 S/cm for lithiated Si; McDowell et al., 2011) can be increased by coating the Si NW with a thin layer of an electrochemically inactive metal such as copper, with a much higher electrical conductivity (5.9 × 105 S/cm). This modification improves cycling efficiency as well as rate capacity by significantly slowing down capacity fading and allowing for controlled expansion of the Cu–Si NW (Chiu et al., 2008; Sethuraman & Srinivasan, 2011). The material
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understanding of the dynamic volumetric changes taking place at the electrolyte/Si NW interface during charge/ discharge cycles. We also discuss modification of the Pt electrodes on the in-situ biasing microchip and the potential problems that one can encounter (and need to overcome) while performing the experiment.
MATERIALS
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METHODS
Assembly of In-Situ Electrochemical Cell
Figure 1. Schematic drawing illustrating the experimental set-up of the nanobattery open-cell approach with ionic liquid as an electrolyte.
becomes more stable and less prone to detachment from the electrode surface during large volume expansion and contraction, which is known as pulverization (Chen et al., 2011b). Moreover, only the LiCoO2 counter electrode is in direct contact with the vacuum compatible ionic liquid electrolyte and only the tip of the Si NW working electrode is within the electrolyte, as shown in Figure 1 (McDowell et al., 2012a). While this design is significantly different from a real battery set-up, where the liquid electrolyte is a medium for three-dimensional lithium (Li)-ion transport between the electroactive surfaces of the anode and cathode materials, it does allow observation of structural effects at the electrodes to mimic the real system. The shortcoming of the “open-cell” is that it does not permit interaction of the electrode with a real liquid electrolyte to be observed. The recent development of an in-situ liquid ec-TEM “closed-cell” overcomes this limitation and enables electrochemical measurements with both electrodes immersed in a “real” high vapor pressure organic electrolyte. The design of this in-situ liquid cell is based upon the initial experiments in this area—the controlled electrochemical deposition of copper nanoclusters on a polycrystalline Au electrode (Williamson et al., 2003; Radisic et al., 2006). More recent applications of this in-situ holder type include observation of lead sulfate nanoparticle growth (Evans et al., 2011), beam-current-induced growth mechanisms of silver nanocrystals (Woehl et al., 2012), dendritic growth with dissolution of lead as a function of charge passed through the circuit (White et al., 2012), and solid electrolyte interphase (SEI) formation during electrodeposition (Sacci et al., 2014). For operando electrochemistry measurements, the important fact is that this design resembles a real battery and allows for direct observation of structural changes during the battery cycling of all three battery components; cathode, anode, and liquid electrolyte. Here we summarize the main parts of an in-situ ec-TEM cell experiment and focus on a fundamental
All the electrochemical experiments were performed with a commercially available Hummingbird in-situ liquid holder equipped with a removable biasing tip (Hummingbird Scientific, Lacey, WA, USA), as shown in Figure 2. The holder design is based on the three O-ring method, which creates a liquid nanochamber that can be introduced into the TEM column (Woehl et al., 2013). The biasing contacts are directly incorporated in the in-situ TEM tip with internal microfluidic channels placed on both sides of the electrochemical cell, which allows liquid to be introduced under a constant flow rate (Fig. 2a). The assembly of this electrochemical cell in the biasing tip is achieved by first placing the Si chip in the bottom of the cell (Fig. 2a) and then placing the Pt biasing chip on top of the spacer chip with three longer Pt electrodes oriented toward the contacts embedded in the in-situ liquid TEM holder (Figs. 2b and 2c). This creates an electrical contact between the Pt electrodes in the in-situ “closed-cell” and the external electrochemical station. For the experiments described here, both chips have a 50 × 200 µm electron transparent window with amorphous 50 nm thick Si3N4 windows from Hummingbird Scientific. To create the liquid chamber, here the bottom Si chip has 500 nm tall spacers located on both sides of Si3N4 window (Fig. 2a). The encapsulation of the electrolyte can be accomplished by first sandwiching the two chips together and then flowing the liquid through the flow tubing. The liquid electrolyte is typically introduced at a constant 5 µL/min flow rate by a microfluidic syringe pump (Chemyx Inc., Stafford, TX, USA). The biasing chip has six Pt electrodes extending into the Si3N4 membrane area, which enable direct observation of electrochemical reactions at the electrode surface (Fig. 2b). Here, an additional offset in liquid thickness is also set by thickness of the electrode material itself, which usually ranges from 25 to 100 nm depending on the type of electrode (Au or Pt). Before in-situ cell assembly, both the spacer and biasing chip are plasma cleaned for 1 min in an oxygen and argon mixture to create a hydrophilic surface. In addition, all connections in the Hummingbird biasing tip and holder were tested to eliminate short circuits since short circuits are a common cause of experimental failure, and can be recognized by an open-circuit voltage (OCV) measurement of 0 V. The assembly is also typically performed in an argon-filled glove box to minimize moisture exposure for the lithium perchlorate salt (LiClO4) (EC:DMC) electrolyte and Li metal. The final assembled in-situ ec-TEM stage is finally tested in a pumping station (Hummingbird Scientific) to ensure the Si3N4 windows are intact and correctly sealed before insertion in the microscope.
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Figure 2. The in-situ liquid ec-TEM cell assembly. a: The bottom silicon chip with two 500 nm tall spacers placed on both sides and parallel to the rectangular 50 nm thick Si3N4 window. The distance between the spacers defines the liquid path inside the in-situ liquid ec-TEM cell. b: Top view of the biasing microchip with six Pt electrodes patterned onto the surface and extending into the viewing area of the 50 nm thick Si3N4 window. The biasing microchip has a set of three longer electrodes, which make an electrical contact with the contacts embedded in the biasing tip allowing for ec measurement at their surface. The reamaining three shorter electrodes are inactive during the experiment. c: During assembly, the biasing chip is facing down into the 1.0 M LiClO4 (EC:DMC) electrolyte solution. d: Top view of the three electrodes extending into the Si3N4 window. Ec-TEM, electrochemical transmission electron microscope.
The liquid thickness was not measured for this particular experiment, but the minimum thickness should be 500 nm, as defined by the spacer material. Nevertheless, the liquid thickness increased due to bulging of the amorphous Si3N4 window, caused by the pressure difference between the sealed in-situ electrochemical liquid stage and the high vacuum inside the column. All the potentiostatic and galvanostatic experiments were performed with a BioLogic SP-200 potentiostat equipped with a low current probe (Bio-Logic, Knoxville, TN, USA) permitting measurement of ultralow, pA current. In-situ TEM experiments of the lithiation of Cu–Si NW were performed with a FEI Titan environmental transmission electron microscope (FEI, Hillsoboro, OR, USA) operated at 300 kV and equipped with Cs-corrected field emission gun, which allows for 0.1 nm resolution in high-resolution transmission electron microscopy mode. In-situ STEM experiments of delithiation of pure Si NW were performed with a FEI Titan aberration-corrected scanning transmission electron microscope (STEM) operated at 300 kV and equipped with CEOS GmbH double-hexapole aberration corrector for the probe-forming lens, which allows imaging with ∼ 0.1 nm resolution STEM mode.
Electrolyte Solutions To prepare a 1.0 M concentration of electrolyte we used 8.67 wt% lithium perchlorate salt (Sigma-Aldrich, Milwaukee, WI), which was first dried overnight at 110°C under vacuum and dissolved in a mixture of ethylene carbonate (EC; BASF, Independence, OH, USA) and dimethyl carbonate (DMC; BASF). Before the in-situ experiment all the solvents were stored and mixed in an argon-filled glove box.
RESULTS
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DISCUSSION
Device Fabrication and Assembly Modification of the in-situ biasing chip was achieved by drop casting the Si NWs suspended in ethanol onto the Pt electrode surface and drying overnight. In order to maintain electrical conductivity during the experiment, a Si NW was Pt welded using a focused ion beam (FIB; FEI). The Pt weld also prevented detachment of the Si NW while flowing the electrolyte inside the in-situ electrochemical cell. The conceptual design of two electrodes making a battery halfcell is demonstrated in Figure 3. The SEM image in Figure 3a shows the biasing chip with six Pt electrodes extending from
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Figure 3. Pt biasing chip modified with Si nanowire (SI NW). a: A scanning electron microscope (SEM) image of the biasing microchip containing the lithium foil (blue) and Si NW; (b) A close-up of the 50 nm thick Si3N4 window with Pt electrodes extended on both sides to allow imaging of the electrolyte/electrode interface; (c–e) SEM images of the Pt electrode surface with welded Si NW drop casted.
the viewing area of the Si3N4 window. The single Si NW in the in-situ electrochemical TEM half-cell was used as a working electrode while Li metal was used as a counter electrode. A small piece of Li foil was mounted on the contacts in the biasing holder (Figs. 2a and 3a) and made an electrical contact with an adjacent Pt electrode to allow effective transport of Li-ions to the Si NW surface. Figure 3c shows an ideal case of a Si NW placed on top of the Pt electrode. The drawback in the drop-casting process is that it provides limited control over the deposition of NWs and as a result, many Si NWs are placed close to the electrode surface (which is illustrated in Figs. 3d and 3e). To address this problem, the Pt weld was extended into the viewing area of the Si3N4 window. Although no results will be shown here, it is worth discussing another extensively studied approach to modifying the electrode surface—the electrodeposition of a Li film. Lithium metal is a very attractive anode material because of its lowest potential and highest theoretical capacity (3,760 mA/g), which is approximately ten times higher than for graphite anode materials (Nishikawa et al., 2007). However, the commercial application of Li metal as an anode is hindered by the formation of Li dendrites during cycling, which leads to short-circuiting, sudden capacity loss, and in some cases, explosion during battery operation (Fu et al., 2006). Electrodeposition has received much attention due to low cost, strong adherence to the substrate, and potential application in a large-scale production. Despite previously stated difficulties, significant progress has been made in understanding the mechanism of dendrite growth during
charge/discharge cycles to obtain dendrite-free electrodeposition. Yang et al. (2005) studied the morphology of Li films during electrodeposition from LiPF6 (EC:DMC) electrolyte on copper substrate under static and dynamic conditions, and found suppressed dendritic Li growth at a high current density of 2.0 mA/cm2. An alternative approach can be taken by adding inorganic additives, such as CO2 (Aurbach et al., 1992; Osaka et al., 1997) and hydrogen fluoride (HF) (Kanamura et al., 1998), to nonaqueous electrolyte to improve cycling efficiency. Recently a novel self-healing electrostatic shield mechanism was proposed where at a low concentration of additive cations a positively charged layer of Cs+ or Rb+ was formed around a sharp edge of a Li deposit. It is well known that a protrusion in the electrode surface exhibits a stronger electrical field than the flat regions, and presence of a cation layer repels Li+ disrupting electrodeposition of Li to adjacent areas of the anode. This process eliminates dendrite growth and leads to a smooth Li deposit in Li metal batteries (Ding et al., 2013). A similar electrodeposition approach can be utilized in the in-situ ec-TEM cell to study different cathode and anode materials and give better understanding of the very complex mechanism of dendrite formation the in-situ ec-TEM cell we use a Li foil as a counter.
Electrochemical Cycling To demonstrate the ability to cycle the in-situ electrochemical cell, here we utilize an Si NW that is commonly
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Figure 4. In-situ liquid-cell TEM observation of the lithiation of the Cu-coated Si (Cu–Si) NW. a: TEM image showing the pristine state of the Cu–Si NW at 0 s; (b) core-shell formation of the Cu–Si NW during lithiation at 1,658 s; (c) TEM image of the Cu–Si NW at 2,462 s; (d) plotted width changes of the NW as a function of time. Note that, in all images from (a–c), the Pt contact region is labeled by the black lines in the left of the image. The inset in (c) illustrating the cross-sectional image after anisotropic swelling of the Si NW upon lithium insertion with maximum volume expansion along the < 110 > direction (Gu et al., 2013). TEM, transmission electron microscope; NW, nanowire.
used in batteries as an anode material as it exhibits high gravimetric capacity, low toxicity, and high abundance. Due to these properties, Si can potentially replace graphite in the next generation of Li-ion batteries (Kasavajjula et al., 2007). However, Si exhibits a large volume expansion upon the first lithiation cycle through the insertion of Li and formation of the crystalline alloy Li15Si4 phase—fully lithiated Si has up to ∼ 300% (4.4 Li atoms/Si; Beaulieu et al., 2003) volume expansion, exhibiting much larger volumetric change than graphite ( direction and possibly includes anisotropic Si and a moderate amount of SEI layer (Fig. 4d). By tracing the Si NW lithiation process in the in-situ Supplementary Movie 1, we can observe a fast initial expansion of Si NW that slows down with time. This behavior is associated with limited diffusion of Li + inside the Si core because of expansion stress on the Si NW during lithiation (McDowell et al., 2012a, 2012b). The delithiation process was performed with a pure Si NW with Pt markers incorporated into the NW using FIB. These markers help to localize the NW in the liquid by
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creating a high contrast area (Figs. 5b to 5d). The initial diameter of the prior lithiated Si NW was ~195 nm as is illustrated in Figure 5b. The initial noticeable decrease in NW thickness was observed around 0.45 V (Fig. 5c) and continues until 0.65 V (Fig. 5d). In the final delithiation stage, the diameter of the Si NW was ~92 nm (Fig. 5d). This indicates that most of the Li-ions were extracted from the Si NW during the delithiation process. The corresponding movie of the delithiation process is shown in Supplementary Movie 2. Supplementary Movie 2 Supplementary Movie 2 can be found online at: http://pubs.acs.org/doi/suppl/10.1021/nl403402q, then accessing the link nl403402q_si_003.avi (65.38 MB). Reproduced with permission from Nano Letters, 2013, 13(12), pp. 6106–6112, MengGu et al., Nov. 13, 2013, Copyright © 2013 American Chemical Society, and the Pacific Northwest National Laboratory.
In both cases, the Cu–Si NW (Fig. 4) and the Pt–Si NW (Fig. 5) are fully immersed in the LiClO4 (EC:DMC) electrolyte, which replicates the real conditions present in a conventional coin cell battery with both cathode and anode in direct contact with the electrolyte solution. It should be noted that the electron dose rates used in the in-situ ec-TEM and ec-STEM experiments were 16.84 electrons/nm2s (Fig. 4) and 5.52 electrons/nm2 s (Fig. 5), respectively.
CONCLUSIONS In conclusion, we describe the modification of a Pt biasing microchip and operation of the in-situ liquid electrochemical cell. We utilize this in-situ liquid ec-TEM holder to track volumetric changes of an Si NW during the lithiation and delithiation processes under a potentiostatic condition with both cathode and anode fully submersed in LiClO4 in EC: DMC liquid battery electrolyte. Current design of the in-situ liquid ec-TEM cell allowed for Li+ insertion from all possible directions and monitoring of volumetric changes of whole Si NW during electrochemical cycling. However, because of the reduced spatial resolution it is difficult to differentiate the amorphous LixSi from the SEI layer under the current imaging conditions and large thickness of electrolyte layer. Future experiments will focus on optimizing the conditions to observe the SEI layer formation.
ACKNOWLEDGMENTS This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the US Department of Energy, Office of Science, Basic Energy Sciences. Development of the electrochemical liquid cell is supported by the Chemical Imaging Initiative at Pacific Northwest National Laboratory (PNNL).
J. Zhang and Jun Liu would like to acknowledge the support of their time by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the US Department of Energy under Contract No. DE-AC0205CH11231, Subcontract No. 18769 under the Batteries for Advanced Transportation Technologies (BATT) program. The work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the Department of Energy under Contract DE-AC05-76RLO1830. The work in Oak Ridge is supported by the Fluid Interface Reactions Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Office of Basic Energy Sciences (BES)DOE (RRU).
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In-Situ Electrochemical Transmission Electron Microscopy for Battery Research B. Layla Mehdi,1,* Meng Gu,2 Lucas R. Parent,1 Wu Xu,3 Eduard N. Nasybulin,3 Xilin Chen,3 Raymond R. Unocic,4 Pinghong Xu,5 David A. Welch,5 Patricia Abellan,1 Ji-Guang Zhang,3 Jun Liu,3 Chong-Min Wang,2 Ilke Arslan,1 James Evans,2 and Nigel D. Browning1 1
Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, 99352, USA 3 Energy and Environmental Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA 4 Oak Ridge National Laboratory, Center for Nanophase Materials Sciences, Oak Ridge, TN 37831, USA 5 Department of Chemical Engineering and Materials Science, University of California-Davis, One Shields Ave, Davis, CA 95616, USA 2
Abstract: The recent development of in-situ liquid stages for (scanning) transmission electron microscopes now makes it possible for us to study the details of electrochemical processes under operando conditions. As electrochemical processes are complex, care must be taken to calibrate the system before any in-situ/operando observations. In addition, as the electron beam can cause effects that look similar to electrochemical processes at the electrolyte/electrode interface, an understanding of the role of the electron beam in modifying the operando observations must also be understood. In this paper we describe the design, assembly, and operation of an in-situ electrochemical cell, paying particular attention to the method for controlling and quantifying the experimental parameters. The use of this system is then demonstrated for the lithiation/delithiation of silicon nanowires. Key words: in-situ liquid ec-TEM, assembly of in-situ liquid electrochemical TEM cell, Li-ion battery, Si anode, Si nanowire lithiation/delithiation, Li-ion battery
I NTRODUCTION The rapidly growing need for new energy storage materials (both electrodes and electrolytes) has created a high demand for experimental techniques that can provide real-time information on the dynamic electrochemical processes taking place at the liquid electrolyte/electrode interface during battery charge/discharge cycles. There are many in-situ techniques that have provided essential insights into the operation of batteries, such as optical microscopy (Beaulieu et al., 2001; Rosso et al., 2006), scanning electron microscopy (SEM) (Orsini et al., 1998; Chen et al., 2011a), X-ray diffraction (Hatchard & Dahn, 2004; Obrovac & Christensen, 2004; Li & Dahn, 2007), nuclear magnetic resonance spectroscopy (Key et al., 2009, 2011; Bhattacharya et al., 2010), transmission X-ray microscopy (Chao et al., 2010), Raman spectroscopy (Hardwick et al, 2008; Long et al., 2011), and neutron diffraction (Bridges et al., 2012). However, most of these techniques focus on examining the structure of either the cathode or anode materials and do not focus on the whole system (i.e., the interface between the liquid electrolytes and electrodes) that is crucial to understanding ion transport and battery performance (Liu et al., 2011; Yu et al, 2012). An in-situ liquid electrochemical transmission electron microscope holder (in-situ liquid Received November 10, 2013; accepted February 20, 2014 *Corresponding author. [email protected]
ec-TEM), on the other hand, offers unique opportunities for monitoring the dynamic processes and structural evolution of varied electrode materials and their interactions with battery-relevant electrolytes during operation at both spatial and temporal resolution (Huang et al., 2010; Wang et al., 2010; de Jonge & Ross, 2011; Gu et al., 2013; Sacci et al., 2014; Unocic et al., 2014), something that is not achievable with the previously mentioned characterization techniques. Open-cell in-situ TEM holders have already made a significant contribution into a fundamental understanding of structural and chemical changes of many electrode materials, in particular, silicon nanowires (Si NW) and tin dioxide (Wang et al., 2010; McDowell et al., 2012a, 2012b). The simple design of a nanobattery open-cell is illustrated in Figure 1. This approach is based on two gold electrodes facing each other, where the working electrode is modified with the nanomaterial of interest, e.g., a Si NW, and the counter electrode is comprised of lithium cobalt dioxide (LiCoO2), which is used to create the full cell. The nominal poor conductivity of the Si NW anode (~1–10 S/cm for lithiated Si; McDowell et al., 2011) can be increased by coating the Si NW with a thin layer of an electrochemically inactive metal such as copper, with a much higher electrical conductivity (5.9 × 105 S/cm). This modification improves cycling efficiency as well as rate capacity by significantly slowing down capacity fading and allowing for controlled expansion of the Cu–Si NW (Chiu et al., 2008; Sethuraman & Srinivasan, 2011). The material
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understanding of the dynamic volumetric changes taking place at the electrolyte/Si NW interface during charge/ discharge cycles. We also discuss modification of the Pt electrodes on the in-situ biasing microchip and the potential problems that one can encounter (and need to overcome) while performing the experiment.
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Assembly of In-Situ Electrochemical Cell
Figure 1. Schematic drawing illustrating the experimental set-up of the nanobattery open-cell approach with ionic liquid as an electrolyte.
becomes more stable and less prone to detachment from the electrode surface during large volume expansion and contraction, which is known as pulverization (Chen et al., 2011b). Moreover, only the LiCoO2 counter electrode is in direct contact with the vacuum compatible ionic liquid electrolyte and only the tip of the Si NW working electrode is within the electrolyte, as shown in Figure 1 (McDowell et al., 2012a). While this design is significantly different from a real battery set-up, where the liquid electrolyte is a medium for three-dimensional lithium (Li)-ion transport between the electroactive surfaces of the anode and cathode materials, it does allow observation of structural effects at the electrodes to mimic the real system. The shortcoming of the “open-cell” is that it does not permit interaction of the electrode with a real liquid electrolyte to be observed. The recent development of an in-situ liquid ec-TEM “closed-cell” overcomes this limitation and enables electrochemical measurements with both electrodes immersed in a “real” high vapor pressure organic electrolyte. The design of this in-situ liquid cell is based upon the initial experiments in this area—the controlled electrochemical deposition of copper nanoclusters on a polycrystalline Au electrode (Williamson et al., 2003; Radisic et al., 2006). More recent applications of this in-situ holder type include observation of lead sulfate nanoparticle growth (Evans et al., 2011), beam-current-induced growth mechanisms of silver nanocrystals (Woehl et al., 2012), dendritic growth with dissolution of lead as a function of charge passed through the circuit (White et al., 2012), and solid electrolyte interphase (SEI) formation during electrodeposition (Sacci et al., 2014). For operando electrochemistry measurements, the important fact is that this design resembles a real battery and allows for direct observation of structural changes during the battery cycling of all three battery components; cathode, anode, and liquid electrolyte. Here we summarize the main parts of an in-situ ec-TEM cell experiment and focus on a fundamental
All the electrochemical experiments were performed with a commercially available Hummingbird in-situ liquid holder equipped with a removable biasing tip (Hummingbird Scientific, Lacey, WA, USA), as shown in Figure 2. The holder design is based on the three O-ring method, which creates a liquid nanochamber that can be introduced into the TEM column (Woehl et al., 2013). The biasing contacts are directly incorporated in the in-situ TEM tip with internal microfluidic channels placed on both sides of the electrochemical cell, which allows liquid to be introduced under a constant flow rate (Fig. 2a). The assembly of this electrochemical cell in the biasing tip is achieved by first placing the Si chip in the bottom of the cell (Fig. 2a) and then placing the Pt biasing chip on top of the spacer chip with three longer Pt electrodes oriented toward the contacts embedded in the in-situ liquid TEM holder (Figs. 2b and 2c). This creates an electrical contact between the Pt electrodes in the in-situ “closed-cell” and the external electrochemical station. For the experiments described here, both chips have a 50 × 200 µm electron transparent window with amorphous 50 nm thick Si3N4 windows from Hummingbird Scientific. To create the liquid chamber, here the bottom Si chip has 500 nm tall spacers located on both sides of Si3N4 window (Fig. 2a). The encapsulation of the electrolyte can be accomplished by first sandwiching the two chips together and then flowing the liquid through the flow tubing. The liquid electrolyte is typically introduced at a constant 5 µL/min flow rate by a microfluidic syringe pump (Chemyx Inc., Stafford, TX, USA). The biasing chip has six Pt electrodes extending into the Si3N4 membrane area, which enable direct observation of electrochemical reactions at the electrode surface (Fig. 2b). Here, an additional offset in liquid thickness is also set by thickness of the electrode material itself, which usually ranges from 25 to 100 nm depending on the type of electrode (Au or Pt). Before in-situ cell assembly, both the spacer and biasing chip are plasma cleaned for 1 min in an oxygen and argon mixture to create a hydrophilic surface. In addition, all connections in the Hummingbird biasing tip and holder were tested to eliminate short circuits since short circuits are a common cause of experimental failure, and can be recognized by an open-circuit voltage (OCV) measurement of 0 V. The assembly is also typically performed in an argon-filled glove box to minimize moisture exposure for the lithium perchlorate salt (LiClO4) (EC:DMC) electrolyte and Li metal. The final assembled in-situ ec-TEM stage is finally tested in a pumping station (Hummingbird Scientific) to ensure the Si3N4 windows are intact and correctly sealed before insertion in the microscope.
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Figure 2. The in-situ liquid ec-TEM cell assembly. a: The bottom silicon chip with two 500 nm tall spacers placed on both sides and parallel to the rectangular 50 nm thick Si3N4 window. The distance between the spacers defines the liquid path inside the in-situ liquid ec-TEM cell. b: Top view of the biasing microchip with six Pt electrodes patterned onto the surface and extending into the viewing area of the 50 nm thick Si3N4 window. The biasing microchip has a set of three longer electrodes, which make an electrical contact with the contacts embedded in the biasing tip allowing for ec measurement at their surface. The reamaining three shorter electrodes are inactive during the experiment. c: During assembly, the biasing chip is facing down into the 1.0 M LiClO4 (EC:DMC) electrolyte solution. d: Top view of the three electrodes extending into the Si3N4 window. Ec-TEM, electrochemical transmission electron microscope.
The liquid thickness was not measured for this particular experiment, but the minimum thickness should be 500 nm, as defined by the spacer material. Nevertheless, the liquid thickness increased due to bulging of the amorphous Si3N4 window, caused by the pressure difference between the sealed in-situ electrochemical liquid stage and the high vacuum inside the column. All the potentiostatic and galvanostatic experiments were performed with a BioLogic SP-200 potentiostat equipped with a low current probe (Bio-Logic, Knoxville, TN, USA) permitting measurement of ultralow, pA current. In-situ TEM experiments of the lithiation of Cu–Si NW were performed with a FEI Titan environmental transmission electron microscope (FEI, Hillsoboro, OR, USA) operated at 300 kV and equipped with Cs-corrected field emission gun, which allows for 0.1 nm resolution in high-resolution transmission electron microscopy mode. In-situ STEM experiments of delithiation of pure Si NW were performed with a FEI Titan aberration-corrected scanning transmission electron microscope (STEM) operated at 300 kV and equipped with CEOS GmbH double-hexapole aberration corrector for the probe-forming lens, which allows imaging with ∼ 0.1 nm resolution STEM mode.
Electrolyte Solutions To prepare a 1.0 M concentration of electrolyte we used 8.67 wt% lithium perchlorate salt (Sigma-Aldrich, Milwaukee, WI), which was first dried overnight at 110°C under vacuum and dissolved in a mixture of ethylene carbonate (EC; BASF, Independence, OH, USA) and dimethyl carbonate (DMC; BASF). Before the in-situ experiment all the solvents were stored and mixed in an argon-filled glove box.
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Device Fabrication and Assembly Modification of the in-situ biasing chip was achieved by drop casting the Si NWs suspended in ethanol onto the Pt electrode surface and drying overnight. In order to maintain electrical conductivity during the experiment, a Si NW was Pt welded using a focused ion beam (FIB; FEI). The Pt weld also prevented detachment of the Si NW while flowing the electrolyte inside the in-situ electrochemical cell. The conceptual design of two electrodes making a battery halfcell is demonstrated in Figure 3. The SEM image in Figure 3a shows the biasing chip with six Pt electrodes extending from
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Figure 3. Pt biasing chip modified with Si nanowire (SI NW). a: A scanning electron microscope (SEM) image of the biasing microchip containing the lithium foil (blue) and Si NW; (b) A close-up of the 50 nm thick Si3N4 window with Pt electrodes extended on both sides to allow imaging of the electrolyte/electrode interface; (c–e) SEM images of the Pt electrode surface with welded Si NW drop casted.
the viewing area of the Si3N4 window. The single Si NW in the in-situ electrochemical TEM half-cell was used as a working electrode while Li metal was used as a counter electrode. A small piece of Li foil was mounted on the contacts in the biasing holder (Figs. 2a and 3a) and made an electrical contact with an adjacent Pt electrode to allow effective transport of Li-ions to the Si NW surface. Figure 3c shows an ideal case of a Si NW placed on top of the Pt electrode. The drawback in the drop-casting process is that it provides limited control over the deposition of NWs and as a result, many Si NWs are placed close to the electrode surface (which is illustrated in Figs. 3d and 3e). To address this problem, the Pt weld was extended into the viewing area of the Si3N4 window. Although no results will be shown here, it is worth discussing another extensively studied approach to modifying the electrode surface—the electrodeposition of a Li film. Lithium metal is a very attractive anode material because of its lowest potential and highest theoretical capacity (3,760 mA/g), which is approximately ten times higher than for graphite anode materials (Nishikawa et al., 2007). However, the commercial application of Li metal as an anode is hindered by the formation of Li dendrites during cycling, which leads to short-circuiting, sudden capacity loss, and in some cases, explosion during battery operation (Fu et al., 2006). Electrodeposition has received much attention due to low cost, strong adherence to the substrate, and potential application in a large-scale production. Despite previously stated difficulties, significant progress has been made in understanding the mechanism of dendrite growth during
charge/discharge cycles to obtain dendrite-free electrodeposition. Yang et al. (2005) studied the morphology of Li films during electrodeposition from LiPF6 (EC:DMC) electrolyte on copper substrate under static and dynamic conditions, and found suppressed dendritic Li growth at a high current density of 2.0 mA/cm2. An alternative approach can be taken by adding inorganic additives, such as CO2 (Aurbach et al., 1992; Osaka et al., 1997) and hydrogen fluoride (HF) (Kanamura et al., 1998), to nonaqueous electrolyte to improve cycling efficiency. Recently a novel self-healing electrostatic shield mechanism was proposed where at a low concentration of additive cations a positively charged layer of Cs+ or Rb+ was formed around a sharp edge of a Li deposit. It is well known that a protrusion in the electrode surface exhibits a stronger electrical field than the flat regions, and presence of a cation layer repels Li+ disrupting electrodeposition of Li to adjacent areas of the anode. This process eliminates dendrite growth and leads to a smooth Li deposit in Li metal batteries (Ding et al., 2013). A similar electrodeposition approach can be utilized in the in-situ ec-TEM cell to study different cathode and anode materials and give better understanding of the very complex mechanism of dendrite formation the in-situ ec-TEM cell we use a Li foil as a counter.
Electrochemical Cycling To demonstrate the ability to cycle the in-situ electrochemical cell, here we utilize an Si NW that is commonly
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Figure 4. In-situ liquid-cell TEM observation of the lithiation of the Cu-coated Si (Cu–Si) NW. a: TEM image showing the pristine state of the Cu–Si NW at 0 s; (b) core-shell formation of the Cu–Si NW during lithiation at 1,658 s; (c) TEM image of the Cu–Si NW at 2,462 s; (d) plotted width changes of the NW as a function of time. Note that, in all images from (a–c), the Pt contact region is labeled by the black lines in the left of the image. The inset in (c) illustrating the cross-sectional image after anisotropic swelling of the Si NW upon lithium insertion with maximum volume expansion along the < 110 > direction (Gu et al., 2013). TEM, transmission electron microscope; NW, nanowire.
used in batteries as an anode material as it exhibits high gravimetric capacity, low toxicity, and high abundance. Due to these properties, Si can potentially replace graphite in the next generation of Li-ion batteries (Kasavajjula et al., 2007). However, Si exhibits a large volume expansion upon the first lithiation cycle through the insertion of Li and formation of the crystalline alloy Li15Si4 phase—fully lithiated Si has up to ∼ 300% (4.4 Li atoms/Si; Beaulieu et al., 2003) volume expansion, exhibiting much larger volumetric change than graphite ( direction and possibly includes anisotropic Si and a moderate amount of SEI layer (Fig. 4d). By tracing the Si NW lithiation process in the in-situ Supplementary Movie 1, we can observe a fast initial expansion of Si NW that slows down with time. This behavior is associated with limited diffusion of Li + inside the Si core because of expansion stress on the Si NW during lithiation (McDowell et al., 2012a, 2012b). The delithiation process was performed with a pure Si NW with Pt markers incorporated into the NW using FIB. These markers help to localize the NW in the liquid by
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creating a high contrast area (Figs. 5b to 5d). The initial diameter of the prior lithiated Si NW was ~195 nm as is illustrated in Figure 5b. The initial noticeable decrease in NW thickness was observed around 0.45 V (Fig. 5c) and continues until 0.65 V (Fig. 5d). In the final delithiation stage, the diameter of the Si NW was ~92 nm (Fig. 5d). This indicates that most of the Li-ions were extracted from the Si NW during the delithiation process. The corresponding movie of the delithiation process is shown in Supplementary Movie 2. Supplementary Movie 2 Supplementary Movie 2 can be found online at: http://pubs.acs.org/doi/suppl/10.1021/nl403402q, then accessing the link nl403402q_si_003.avi (65.38 MB). Reproduced with permission from Nano Letters, 2013, 13(12), pp. 6106–6112, MengGu et al., Nov. 13, 2013, Copyright © 2013 American Chemical Society, and the Pacific Northwest National Laboratory.
In both cases, the Cu–Si NW (Fig. 4) and the Pt–Si NW (Fig. 5) are fully immersed in the LiClO4 (EC:DMC) electrolyte, which replicates the real conditions present in a conventional coin cell battery with both cathode and anode in direct contact with the electrolyte solution. It should be noted that the electron dose rates used in the in-situ ec-TEM and ec-STEM experiments were 16.84 electrons/nm2s (Fig. 4) and 5.52 electrons/nm2 s (Fig. 5), respectively.
CONCLUSIONS In conclusion, we describe the modification of a Pt biasing microchip and operation of the in-situ liquid electrochemical cell. We utilize this in-situ liquid ec-TEM holder to track volumetric changes of an Si NW during the lithiation and delithiation processes under a potentiostatic condition with both cathode and anode fully submersed in LiClO4 in EC: DMC liquid battery electrolyte. Current design of the in-situ liquid ec-TEM cell allowed for Li+ insertion from all possible directions and monitoring of volumetric changes of whole Si NW during electrochemical cycling. However, because of the reduced spatial resolution it is difficult to differentiate the amorphous LixSi from the SEI layer under the current imaging conditions and large thickness of electrolyte layer. Future experiments will focus on optimizing the conditions to observe the SEI layer formation.
ACKNOWLEDGMENTS This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the US Department of Energy, Office of Science, Basic Energy Sciences. Development of the electrochemical liquid cell is supported by the Chemical Imaging Initiative at Pacific Northwest National Laboratory (PNNL).
J. Zhang and Jun Liu would like to acknowledge the support of their time by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the US Department of Energy under Contract No. DE-AC0205CH11231, Subcontract No. 18769 under the Batteries for Advanced Transportation Technologies (BATT) program. The work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the Department of Energy under Contract DE-AC05-76RLO1830. The work in Oak Ridge is supported by the Fluid Interface Reactions Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Office of Basic Energy Sciences (BES)DOE (RRU).
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